The Functional Significance of the MPL Expression in the Primitive Human Hematopoietic Stem Cell Compartment

Abstract

Thrombopoietin (TPO) was first identified as a cytokine that promotes megakaryocyte proliferation, maturation and platelet production. In the murine hematopoietic system, the TPO receptor MPL is expressed not only in megakaryocyte progenitors and their progeny but also in hematopoietic stem/progenitor cells (HSPCs). In addition, in the murine hematopoietic stem cell (HSC) compartment, TPO/MPL signaling plays an important role in the maintenance of adult quiescent HSCs. However, the role of TPO/MPL signaling in the human primitive HSC compartment has not yet been elucidated. Previously, we identified CD34-negative (CD34^-) SCID-repopulating cells (SRCs) in human cord blood using an intra-bone marrow injection technique. In addition, we clearly demonstrated that human CD34^+/- HSPCs expressed MPL via flow cytometry. Therefore, in order to investigate the role of the MPL expression in the human primitive CD34^+/- HSC compartment, we performed a serial (primary, secondary and tertiary) transplantation analysis of CD34^+/-MPL^+/- cells using NOG mice for over a year. Our results revealed that all four fractions of cells had primary human cell repopulation abilities. However, only the CD34^+/-MPL^- cells showed tertiary human cell repopulation abilities. These results clearly indicate that the CD34^+/- SRCs not expressing MPL sustain long-term (> 1 year) human cell repopulation in NOG mice. Collectively, these findings highlight a new concept that CD34^-MPL^- SRCs (HSCs) reside at the apex of the human HSC hierarchy.